I'm looking to upgrade my current build which is starting to show its age and would like some recommendations as I've been out of the loop for a few years are there now a ridiculous amount of choice that I'm a little lost amongst the reviews.

But if you wish to update the video card, an ATI 7770 is a good mid range cards.

If you do CAD, you may want to look at Intel, but it is expensive.. So if you stay on AMD, get an FX-8320 - fast 8 core CPU, I bought one for video encoding and was so impressed bought another one for sister machine..
Board wise look at the 970 or 990 AMD chipset.. If you had no issues with Gigabyte, they are pretty good. ASRock is very good too as are the more expensive ASUS (I have two ASRock 870 Extremes R2 and an ASUS M5A99 Evo, rock solid).
As for the operating system, ignore all the hype and also the moaning, Windows8 is OK.. As good if not quicker than 7, just takes a bit of getting used to, but really does the job..

If your budget is £600 and you can keep the Case, PSU, RAM, hard drive (s)/DVDRW (proving they are not IDE) you should get a rocking system and have enough for a solid state drive you can run windows off...

I have been programming on computers since the ZX81.
I am an apprentice trained Electronics Engineer with qualifications to back it up.
I have been repairing computers since 1996.
Yet to some people I still know nothing...

Only really looking to upgrade the CPU, motherboard and graphics card, as everything else is working fine, although I may need to get some new hard drives because my current ones are nearly 4yrs old and will probably be close to dying.

I've always had AMD's because I like the underdogs but I'm tempted by the intels for the performance and to future proof the machine a little, any recommendations for "bang for bucks" on intels? for comparisons sake. I'll read up on processor you recommended and graphics card.

Per this specs link to that Gigabyte mobo, it has dual-channel memory design. Yes, each channel should have the same amount of memory and type for best performance and stability. If one channel has more or less memory than the other, the system may be unstable or not performing optimally.
And per the notation about memory setup, any amount over 4 GB may not be addressed.

(Note 1) Due to Windows Vista/XP 32-bit operating system limitation, when more than 4 GB of physical memory is installed, the actual memory size displayed will be less than 4 GB

For a 32-bit OS that can address no more than 4 GB of system memory, that must include any devices having their own dedicated memory.. such as a video card. A 1 GB dedicated memory video card ? Then only about 3 GB maximum system memory available!

i'm curious how your ram worked in that motherboard with odd nmber of sticks. i was under the assumption you needed even numbers of sticks of ram if you went over 1.

I actually only have two sticks installed because the CPU cooler I have on the board overhangs the first set of memory sockets, the list I was quoting is what I have in my records for what I bought the stuff (dangers of copy and paste).